The present invention generally relates to methods and apparatus for forming optical fibers and, more particularly relates to an optical fiber production method for forming and cooling optical fiber.
Conventional manufacturing processes for producing optical fibers typically include drawing an optical fiber from an optical fiber preform in a draw furnace, cooling the drawn fiber, and coating the fiber after it has sufficiently cooled. The optical fiber is typically drawn in a furnace at about 2,000° C. and the heat is typically transported to the preform mostly by radiation, but the flow of gas in the furnace, the result of forced flow from blanketing and natural convection, can also affect the glass temperature. The relative contribution of convective heat transfer is significant at the lower part of the fiber forming zone, the region at the optical fiber preform root and below, where the radiative heat transport becomes negligible due to the small diameter of the fiber.
The gas flow in the furnace, resulting from forced and free convection, typically creates convection cells, and these cells can become unstable under certain conditions of temperature gradient and gas density. This unsteady motion affects the heat transfer in the fiber forming zone enough that the fiber clad diameter can vary significantly, which is generally undesirable. To counteract this effect, helium may be used as the gas in the furnace. Helium reduces the strength of the convection cells and the temperature difference across the cells. This typically results in improved fiber diameter control, but the disadvantage is that helium, which is expensive, is consumed. Additionally, the high temperatures used in the furnace can potentially create defects, typically density fluctuations, in the core of the fiber, the numbers of which are distributed according to thermal equilibrium (e.g., Boltzmann distribution). Defects in the fiber structure usually represent states with higher energy, so the number of defects is typically greater at higher draw temperatures. Such defects in the fiber can introduce signal loss in the optical fiber. To reduce this increased attenuation, it is desirable to cool the fiber slowly, especially between temperatures of 1,600° C. and 1,300° C., to allow the defects time to heal before increasing viscosity of the solidifying glass freezes the defects in. In this temperature range, radiative cooling is negligible, so one can effectively reduce cooling rate by reducing the temperature difference between the fiber and the gas in which it is immersed.